Recently, two-part type adhesives called "second-generation acrylic adhesives" have been developed. These adhesives are usually composed of the combination of an acrylic manner, an elastomer, an organic peroxide, and a curing accelerator and these adhesives are generally classified into the following two kinds according to the kinds of the elastomers used. One of the adhesives contains chlorosulfonated polyethylene as the elastomer and has a mechanism of forming a radical at the side chain of the elastomer at curing and an acrylic monomer graft-polymerizes to the radical (e.g. U.S. Pat. No. 3,890,407). This type of adhesive has excellent adhesive property but, on the other hand, has the fault that chlorine is released from the adhesive by preserving the adhesive for a long period of time or by heating it at high temperature since chlorosulfonated polyethylene is used as the elastomer and the adhered surface of a metal is corroded by the action of chlorine to reduce the adhesive strength. For avoiding the occurrence of the fault, it is attempted to add an epoxy compound as a dechlorinating absorbent to stabilize the adhesive but the sufficient effect is not obtained. Another type of adhesive contains a diene series elastomer such as a nitrile rubber, etc. (e.g. U.S. Pat. No. 3,832,274), but the chemical bonding of the diene series elastomer and an acrylic monomer is insufficient as compared to the case of using chlorosulfonated polyethylene and the adhesive strength is not satisfactory.
The inventors made various investigations for developing high-quality novel two-part type acrylic adhesives without accompanying with the foregoing faults in conventional techniques on the second-generation acrylic adhesives. That is, since when a nitrile rubber having no chlorosulfone group was used as an elastomer which was one of the components of the adhesives and methacrylic groups or acrylic groups (hereinafter, is referred to as (meth)acrylic group) were introduced into the molecule of said nitrile rubber, the grafting reaction or the partial crosslinking reaction certainly occurred by adhesion and also dechlorinating reaction by heat decomposition did not occur, the inventors considered that the adhesives had good adhesiveness and good heat resistance. For introducing (meth)acrylic groups into the molecule, a method by an ionic bond was first employed.
As acrylic adhesives having ionic bond, there are one-part type anaerobic adhesives previously proposed by the inventors (see, U.S. Pat. Nos. 3,899,382 and 3,933,748). These adhesives had excellent adhesive strength and adhesion for oily surfaces owing to the effect of the ionic bond but since the cured product thereof was lacking in flexibility, the adhesives were unsuitable as the adhesives for structural uses.
Thus, the inventors turned our attention to a nitrile rubber having a good flexibility and carboxyl groups in its molecule, and obtained the modified nitrile rubber having the desired (meth)acrylic groups in its molecule by neutralizing the carboxyl groups of the nitrile rubber with a metal compound or an amine in the presence of a compound having a (meth)acrylic group and an acid group in its molecule. And it was found that the two-part type acrylic adhesive prepared by using the nitrile rubber thus obtained showed excellent properties.
In this case, it was also found that when a (meth)acrylic group-containing acid phosphate shown by the general formula (I) ##STR1## (wherein R represents a hydrogen atom or a methyl group; R' represents an alkylene group having 2 to 4 carbon atoms; m is an integer of 1 to 8; and n is 1 or 2) was used as the compound existing in the foregoing neutralization, the adhesive obtained showed very excellent adhesive strength. However, as the results of applying a long water resisting test and a weather resisting test to the adhesive, there was a tendency of reducing the adhesive strength to some extent as the case of the conventional nitrile rubber series second-generation acrylic adhesives. Therefore, this type of adhesives were also dissatisfactory according to the using purposes.
Therefore, it was investigated to introduce a (meth)acrylic group not only through an ionic bond but also through a covalent bond. As the result of various investigations for introducing a (meth)acrylic group through a covalent bond, it was succeeded in introducing a (meth)acrylic group into the foregoing carboxyl group-containing nitrile rubber through an amide group by addition condensing the carboxyl group of the nitrile rubber and the isocyanate group of the isocyanate group-containing (meth)acrylate. However, when the amount of the isocyanate group-containing (meth)acrylate was over 0.2 equivalent per equivalent of the carboxyl group in the nitrile rubber, a gelation sometimes occurred during the reaction according to the compositions of the reactants and hence it was difficult to introduce a large content of a (meth)acrylic group through a covalent bond.